Heres what you do upgrade.
All your configuration is in the /etc/ folder.
Unfortuanatly I don't know much about sendmail so I can't advice on the actual setup, so it's kinda generic advice.
however 8.x seems pretty new. So what you do is make a copy of your configuration files for sendmail. Check and make sure that the configuration file will work properly with the newest version. After you made the configuration changes you need to make it work with the newest version save a copy of it.
Then when you reinstall everything simply copy the configurations back to the correct location and restart your services, this should make it set up the same as before.
Since everything is stored in plain text files, instead of something weird like the windows registry, all you have to do is do research and find out what configuration files you need to save.
Hell you could probably just copy the entire /etc/ folder from one OS to another and most everything would still work OK, but I wouldn't bet on it. Redhat 6.2 is pretty ancient, so you probably copy each config file on one on one basis.
What I would do personally to make sure that I don't screw anything up is to get a second harddrive. That way if I mess up on the conversion I can always go back to the original. That seems like the easiest and safest thing to do.
Harddrives are so cheap, you can get a nice one for like 50 bucks from newegg. Seems like cheap insurance. That should make a upgrade a non-stressfull experiance and give you a chance to perfect the new install before making it a production machine.
<a target=new class=ftalternatingbarlinklarge href="http://
http://www.sendmail.org/virtual-hosting.html">If you look here, here is a overview on how to do that virtual hosting stuff.</a>. I know that sendmail can be a beast to work with. It may be a good idea to invest in a book like the one on that page. That way you can use it as a referance material for all your sendmail needs.
I am not experianced personally with sendmail, so that's about the best advice I can think of.